"Nobody has ever managed to keep this mineral stable at the Earth's surface," study co-author Graham Pearson, a professor in the University of Alberta's Department of Earth and Atmospheric Sciences, said in a statement. "The only possible way of preserving this mineral at the Earth's surface is when it's trapped in an unyielding container like a diamond."

In the new study, Pearson and his colleagues analyzed a tiny diamond (roughly 3 millimeters across) excavated from Cullinan less than 1 km (0.6 miles) below the Earth's surface. Despite this relatively shallow depth, the researchers determined that the crystal was an example of a "deep diamond" that most likely had been formed about 700 km below the Earth's surface, derived from a subducted slab of ocean crust and exposed to some 240,000 atmospheres of pressure.

The chunk of calcium silicate perovskite within the gemstone was visible with the naked eye after the diamond was polished, the researchers wrote, but proper analysis and imaging required an international effort. X-ray and spectroscopy tests confirmed that the diamond did contain calcium silicate perovskite — quite possibly the first intact sample ever seen.

"Diamonds are really unique ways of seeing what's in the Earth," Pearson said. "And the specific composition of the perovskite inclusion in this particular diamond very clearly indicates the recycling of oceanic crust into Earth's lower mantle. It provides fundamental proof of what happens to the fate of oceanic plates as they descend into the depths of the Earth."